1. Field of the Invention
The present invention relates to inkjet ink compositions comprising a pigment and a polymer dispersant having attached chromophores.
2. Description of the Related Art
An inkjet ink composition generally consists of a vehicle, which functions as a carrier, and a colorant such as a dye or pigment. Additives and/or cosolvents can also be incorporated in order to adjust the inkjet ink to attain the desired overall performance properties.
In general, pigments alone are not readily dispersible in liquid vehicles, and a variety of techniques have been developed that can provide stable pigment dispersions useful in applications such as inkjet printing. For example, dispersants can be added to the pigment to improve its dispersibility in a particular medium. Examples of dispersants include water-soluble polymers and surfactants. A wide variety of polymers have been used as dispersants, and these are often tailored to the type of pigment to be dispersed. Typically, polymeric dispersants have a molecular weight less than 20,000 in order to maintain solubility and to provide pigment stability. For example, JP 10-130554 describes inkjet ink compositions comprising substituted quinolonoquinolones which further comprise a rosin, resin, surfactant, or dispersant, such as a three-component copolymer composed of methyl methacrylate, ethyl acrylate, and methacrylic acid.
Non-polymeric materials can also be used as dispersants for inkjet inks. For example, U.S. Pat. No. 5,750,323 describes a solid particle aqueous dispersion of a colorant dispersed using a relatively small amount of a compound that is structurally similar to the colorant. This structurally similar additive is structurally distinct from the colorant and contains an identical structural section making up at least 75% of the total molecular weight of the colorant. The additive has at least one substitutent bonded to the identical structural section that has a molecular weight higher than the corresponding substituent of the colorant. However, such additives are described for non-pigment colorants (i.e., filter dyes).
In addition, U.S. Pat. No. 5,716,435 describes a water-dispersed inkjet recording liquid prepared by a salt-milling method in which a mixture containing an organic pigment, a water-soluble inorganic salt, and a water-soluble solvent is mechanically kneaded. A pigment derivative, which is a substituted derivative of a pigment residue or heterocyclic ring residue, or a resin, which is a polymeric dispersant, may also be included. However, such a composition requires the use of both the pigment derivative and polymeric dispersants as separate additives, each of which may be affected by other components in the inkjet ink composition.
Dispersants having pigment derivatives attached to a polymeric group have also been described. For example, GB 2036779 describes polyether disazo dyestuffs having specified formulas which includes a disazo dye and an attached polyalkylene oxide group. These dyestuffs are useful for dying and printing synthetic fibers. Also, JP 63-175080, JP 06-065521, JP 07-041689, and JP 2993088d each describe pigment compositions comprising a pigment and a polymer having an attached quinacridone derivative, which can be used for dispersing a pigment for coatings or varnishes. However, none of these references teaches the use of such additives for the demanding requirements of inkjet ink compositions.
Methods for the preparation of modified pigment products have also been developed which can provide a pigment with a variety of different attached functional groups. For example, U.S. Pat. No. 5,851,280 discloses methods for the attachment of organic groups onto pigments including, for example, attachment via a diazonium reaction wherein the organic group is part of the diazonium salt. Other methods to prepare modified pigments, including those having attached polymeric groups, have also been described. For example, PCT Publication No. WO 01/51566 discloses methods of making a modified pigment by reacting a first chemical group and a second chemical group to form a pigment having attached a third chemical group. These methods provide modified pigments having attached groups and pigment compositions, including inkjet ink compositions, with improved overall performance properties that do not require the addition of dispersant. However, a pigment modification step is needed.
As the inkjet printing industry moves towards print performance similar to that of laser printing, there remains a need for inkjet ink compositions comprising pigments and a dispersant having improved properties, such as improved stability, thereby providing alternatives to modified pigment dispersions.